Material handling vehicles such as fork lift trucks have had various types of construction. In a typical type of vehicle, a drive unit is provided, usually including an electric motor, for causing travel of the vehicle from one point to another, a steering mechanism is provided to control direction of travel, a fork lift mechanism is provided, usually including a hydraulic actuator, and an operator's platform and associated controls are provided, for control of travel, steering and the vertical position of the fork lift. A deadman control is generally provided for disabling the controls and operating a brake in the absence of an operator on the operator's platform.
The Cook et al. U.S. Pat. No. 3,280,401 shows one specific type of construction in which a single manually operable control element controls vehicle braking, travel speed and forward and reverse travel direction. The control element is connected to a master fluid actuator which is connected to a brake actuator to release brakes when moved to or beyond a "coast" position and which is also connected to an actuator for compressing a variable resistor of the carbon-pile type connected in circuit with drive motors of the vehicle. The master fluid actuator is also connected to a plurality of pressure responsive switches for controlling operation of the drive motors in different speed ranges. The brake actuator operates against the force of brake-engaging springs. A solenoid-operated valve is connected through a deadman switch to a power source to be operated to a closed position when an operator stands on his platform and to be operated to an open position to by-pass the brake actuator when no operation is present, thereby causing the spring to apply the brake.
In another type of construction, a pedal is provided on which the operator must stand to close an electrical circuit and to apply power to travel, steering, lift and other functions. The pedal is connected to a master cylinder coupled to a slave cylinder which operates against the force of a brake-applying spring to release a brake when an operator stands on the pedal. In this type of construction, the operator's platform may be vertically movable along with a fork and a suitable cable and sheave linkage is provided between the pedal and the master cylinder located in the main frame of the vehicle. In the alternative, the master cylinder is located on the operator's platform and a reeled hose arrangement is provided between the master cylinder and a slave cylinder which is located on the main frame of the vehicle. A power steering assembly may be provided including a pump driven by an electric motor and coupled to a hydraulic actuator connected mechanically to a drive unit which is turnable about a vertical axis and which journals a wheel driven from an electric drive motor of the drive unit. The steering actuator is controlled from a steering wheel on the operator's platform, through a suitable cable and sheave arrangement allowing vertical movement of the platform.
The construction of the Cook et al. U.S. Pat. No. 3,280,401 also includes a power steering system having a pump driven by an electric motor. The Shaffer U.S. Pat. No. 3,976,158 discloses a specific type of power steering mechanism for a fork-lift truck with pressure accumulating means being provided in a manner such as to reduce the frequency of energization of a drive motor for a power steering pump.
There are many other disclosures of power steering systems and drive control and braking systems for various types of vehicles. The Wurst U.S. Pat. No. 3,870,119 discloses a brake control for preventing creep in an automobile or other vehicle having an automatic transmission, vacuum from the engine of the vehicle being used to develop an operating force for application of a brake.
Such material handling vehicles and such power steering, drive control and braking systems have been generally satisfactory for the purposes for which they have been designed and there are problems which have not been recognized and dealt with in the prior art. One unrecognized problem is encountered with respect to the deadman controls of material handling vehicles. In such controls and particularly in the aforementioned type of construction in which the operator stands on a pedal, the required actuating force is quite large. As a result, when the vehicle is operated for a substantial length of time, the operator can become fatigued from holding the pedal depressed. This problem is especially acute with the operators who are light in weight since the pedal pressure can become quite high, from twenty to forty pounds, for example, as when high performance wet disc brakes are used.
Another problem associated with the deadman control is in connecting the deadman control on a movable platform with a braking mechanism on a drive unit associated with the main or base frame of the vehicle. To provide a direct mechanical connection, suitable cable and sheave linkages may be provided between the pedal and a master cylinder located in the main frame of the vehicle, as aforementioned. As also aforementioned, a reeled hose arrangement may be provided between a master cylinder located on the operator's platform and a slave cylinder located on the main form of the vehicle. With such arrangements, problems can develop with wear and tear on the cable and sheave linkages and reeled hose arrangements, after repeated raising and lowering of the operator's platform.